Implanted "Pacemaker" devices are now commonly employed for the long-term treatment of atrio-ventricular (A-V) block. Such pacemaker devices commonly employ flexible leads which connect a remotely positioned power pack with electrodes which are placed in contact with or attached to the myocardium. The techniques of implanting and using such pacemakers, and many pacemakers which have been used experimentally and in practice, are described in Siddons and Sowton, Cardiac Pacemakers (1967), published by Charles C. Thomas, Springfield, Illinois, Library of Congress Card No. 67-12042. Pacemakers having energy sources responsive to heart movement are shown in U.S. Pat. Nos. 3,358,690 and 3,486,506.
Such Pacemakers, or other biological stimulators working on these principles, have inherently suffered from certain disadvantages. The leads to the electrodes are commonly routed through veins leading into the heart itself. The movement of the heart and normal activity of the individual tend to put a strain on these leads and may result in lead breakage or dislodgement of the electrodes. The leads themselves, retained in situ, are frequently a source of irritation and infection. Further, since the electrical contact with the heart is made at the point or region of mechanical support or implantation, the normal fibrosis of tissue at these regions often results in marked increase of power required to pace, known as an increase in threshold. For example, the threshold has been found to increase on the order of ten times its original value until a plateau is reached over a period of two to three weeks. This requires a correspondingly greater power input to the electrodes, in the minimum of 3:1 over threshold, in order to achieve consistent pacing.
The remote power pack itself is a cause of discomfort and often a cause of difficulty. It is commonly implanted in a subcutaneous pocket beneath the pectoralis major or within the abdomen. Again, this provides a further opportunity for infection. Difficulty has also been encountered in preventing migration of the power pack. Further, surgery is required from time to time to expose and replace the power pack due to exhaustion of the mercury cells.
Prior pacing devices which derive their energy from the heart movement or pressures have commonly required thoracic surgery for attachment to the epicardium, and have employed flexible leads to the electrodes.